In Chapter One, we saw that the digestive tract is one long continuous tube running through the body from mouth to anus, and that the inside of that tube is outside of the body, with the gut wall separating the two. We followed food through the stomach and are set to enter the small intestine. Here again is our map:
A reminder that even when not specifically mentioned, we should imagine water as part of the discussion, as the major component of blood and of the interstitial fluid and other body fluids, the fluid outside the blood vessels.
The small intestine is the major digestion and absorption site and is divided into three sections – the duodenum, the jejunum, and the ileum. The jejunum is the longest section of the small intestine. The differences among these three sections involve some differences in internal responsibility and function but they would not be evident by looking from the outside. Disease of the small intestine tends to be defined as a whole, not necessarily specific to one of its parts.
As a tubular organ, the inner space of the small intestine is called the lumen. Lumen is not the term for the gut wall but for the space inside the gut.
The small intestine is long, about two and a half times the length of the cat on average. Not every cat is average and the individual length may vary. This length is cleverly arranged to fit in a smaller space. The 'map' above shows the small intestine as a folded affair in order to indicate relative length in relationship to the cat. For clarity, other illustrations may show the small intestine looking more like a string of sausage, as in the first illustration below.
In fact the small intestine is a creatively ruffled affair, as we see more clearly in the second image, with the arrangement of the rufflings and riffles of the small intestine varying from individual to individual.
In the third, a photo of cat small intestine with the small intestine partially unruffled, we now see evidence of the mesentery, a membrane that anchors the small intestine to the back wall of the abdominal wall cavity. The mesentery is an elegant evolutionary development, a membrane that not only anchors the small intestine yet still allows flexibility of the organ plus provides pick-up-and-delivery service for all those nutrients that are being released from the food.
How is this design trick accomplished? The mesentery is created by an ingenious folding of the peritoneum. And what is the peritoneum? As skin covers the outside of the body, the peritoneum lines the inside of the abdominal cavity. We could compare the peritoneum to the inner lining of a storm coat, except it does not zip out. Instead it folds itself in remarkable ways during embryonic development, while the fetus is forming.
We might call this folding process the original origami. Nature does a lot of folding!
Each of the origami cats above is made of a single bill, even the ears and tail are accomplished by complex folding without cutting the bill into pieces. The peritoneum folds in similar fashion to solve various design challenges.
Below are two illustrations of the mesentery which show the mesentery's physical connection to the small intestine along with the blood vessels which pick up and transport nutrients throughout the body after digestion in the gut. The mesentery is constructed of a double fold of the peritoneum which wraps around the small intestine and, just as the trailing edge of a ruffle is anchored to a neckline or a sleeve cuff, this mesenteric arrangement is anchored to the back wall of the abdominal cavity. The small intestine is secured yet able to give and stretch as chyme flows through its lumen.
These illustrations show only a small length of small intestine with its corresponding mesentery so a more complete understanding is presented later in this chapter. Here it is enough to know that the small intestine is not flopping around loose inside one's cat.
Immediately after the chyme enters the duodenum, the first part of the small intestine, the acid from the stomach is neutralized by a buffer, a bicarbonate, delivered from the pancreas via the pancreatic duct. Only the stomach is designed to handle its strong gastric acid, not the esophagus nor the small intestine.
Enzymes from the pancreas are delivered to the duodenum to digest carbohydrates, fats, and to complete the digestion of protein. Different enzymes are required to break down different substances, they are specialists. Cells in the villi of the gut wall also synthesize enzymes which assist with breakdown and absorption of various nutrients.
Bile also has neutralizing effects on the stomach acid. Bile, synthesized by the liver and stored in the gall bladder, enters into the duodenum via the bile duct to emulsify edible fats present in the chyme, resulting in a milky mix called chyle. The detergent action of bile breaks down larger fat molecules into smaller molecules and allows the fats to mix with watery lymph which gives greater access to the lipases, fat digesting enzymes from the pancreas. Mix emulsified fats with lymph in the gut and we have chyle. Bile itself does not become a part of the chyle that is later absorbed into the body.
Lymph fluid starts as interstitial fluid, the body's fluid outside the blood circulatory system and outside the cells, surrounding all the cells. Once interstitial fluid enters the lymph system, it is by definition lymph fluid and its composition changes as it flows along through its own circulatory system.
The neutralized chyme flows through the small intestine like a slow swirling river, propelled along by both peristalsis and segmentation. These are different but complementary actions which move chyme along (peristalsis) while simultaneously mixing it (segmentation). We can see why specialized cells such as the Interstitial Cells of Cajal, the ICCs, are needed to help coordinate all these movements. These two actions maximize the access of enzymes to their molecular targets and also maximize contact with the villi and absorption into the body. As we read above, the small intestine has a rich blood supply; blood vessels and the lymph system are the transport systems of the body, the pick-up and delivery.
As the chyme swirls and flows through the jejunum, molecules of digested nutrients contact the villi and are absorbed via the mesentery into the blood system for distribution to all parts of the body where they are put to use.
In the last section of the small intestine, the ileum, Vitamin B12 from meat sources is absorbed.
Some bile salts are resorbed back into the body in the ileum for recycling. Remember that permissible traffic can go both ways through the gut wall barrier.
Other nutrients which were not absorbed in the jejunum get a last chance in the ileum where additional enzymes are provided by cells in the villi.
A common theme develops. Digestion breaks bigger pieces into smaller and smaller pieces. Mechanical digestion uses physical means to break down big pieces, for greater surface area and better enzymatic access. Chemical digestion employs enzymes to snip into smaller molecules by breaking chemical bonds that hold compounds together. Food needs to be broken down before it can cross the gut wall barrier into the blood stream.
That illustration above, of the mesentery, is a view from the outside, not inside the tube we call the gut or digestive tract, not in the lumen. Since the gut wall forms an active barrier between the lumen and the body, to regulate what crosses the barrier, how do those digested food molecules manage to get into the blood stream for transport inside the body? The food is digested but how does it get into the body to nourish the cat? Here is Nature's solution and once again folding is involved, this time of the inner lining of the small intestine into folds called plicae circulares.
On and between these folds are millions of tightly packed little fingerlike projections called villi, more so in the jejunum than in the duodenum. Villi increase surface area manyfold. If a human small intestine were unfolded to flatten all the villi and microvilli, it would cover more area than a football field!
The villi are in turn covered with microvilli. This greatly increased surface area and the richness of the blood supply allow more nutrients to be absorbed without the need to make the small intestine even longer. Folding is remarkably useful.
Not only do the villi and their microvilli expand the inner surface area of the small intestine, they also serve as loading docks and gateways.
If we were looking at a patch of villi with our naked eye, it would look as Henry Gray described it in his Gray's Anatomy, as having "a velvety appearance." That poetic description indicates just how thickly populated the wall of the intestine is with villi, like microfiber only more so. Another term for a surface covered with villi and microvilli, equally descriptive, is 'brush border'. There may be 6,000 to 25,000 villi per square inch of intestinal wall, depending on the species whose intestine it is and the particular section of the intestine. Think lush! The numbers per square inch decrease as we move downstream in the intestine and are thickest in the jejunum where most nutrient absorption takes place.
Here is a greatly magnified view which gives a hint of the microvilli covering the villi.
Gray's Anatomy, a text devoted to human anatomy, provides an illustration of cat intestinal villi including the mucosa and muscle layer of the wall. I have added the coloring which should be seen as representative only. The villi may look tall in these graphics but they are only about 0.5 to 1 millimeter (mm) in length. A postage stamp is about 0.1 millimeter thick. Remember the villi carpet the lining of the small intestine, they are the lining of the small intestine, and all this folding greatly expands the effective surface area of the small intestine. Nature does a lot of folding.
That graphic gives us a hint of the complexities, a suggestion that the little villi have something going on under their surface.
Another of Gray's illustration, a cutaway, reveals more with evidence of the plumbing of the individual villi, their blood and lymphatic vessels. Again the coloring is mine.
Just as the theme in digestion is to break big bits into smaller and smaller bits, a circulatory system has a similar theme, working from big through little, littler and littlest, down to the microcirculation level, and circulating back up through small to big again. Here is a representation of an arrangement of a circulatory system in one direction.
Change the color, turn the image upside down, mirror it, and what do we have?
Once Nature has a good system, it is used and reused and used yet again under a variety of circumstances. A circulatory system functions as a pick-up-and-delivery system; the system can pick up, transport, and deliver useful and necessary goods or pick up, transport, and dump the waste. Humans use similar graduated systems to deliver water and electricity to dwellings and to remove waste.
The smallest blood vessels are not built to withstand the initial pressure of a heartbeat sending blood coursing through the arteries so blood pressure in the little capillaries is much less than blood pressure in the major arteries. Capillaries connect the smallest arteries, the arterioles, which carry oxygenated blood away from the heart, to venules, the smallest veins, which carry blood back to the heart for reoxygenation. Capillaries also facilitate absorption into lymph vessels. The walls of the microscopic capillaries are only one cell layer thick, an important feature since capillaries are where the action is, where exchanges are made. Capillaries are the key to absorption of nutrients into the circulatory systems.
Below we have a representation of two networks intersecting, meant to illustrate connections between the arteries and the veins. Imagine capillaries at all the points where those two touch. If you have trouble imagining capillaries, think of your household wiring system which is hidden behind the walls but appears periodically at outlets and switches and sockets, where you can access and interact with the electricity.
Now back to the villi. As we saw in Gray's illustration above, arteriols (the tiniest arteries) and venules (the tiniest veins) meet in each villi. But just like a household which has electrical wiring, plumbing and waste disposal, there are other systems in the body that need to be incorporated. In the case of the villi, we need to incorporate the lymph system and also take into account the enteric nervous system.
Here is a graphic of a single villus for further insight into its inner workings. Each part of the structure has an important and active function. Remember there are millions of these little villi and each villi is 'wired' into the body's blood circulatory system.
As we see, the outer surface of the villi consists of absorptive cells interspersed with goblet cells. Goblet cells emit mucin which in contact with water forms mucous. Mucin is composed of glycosylated proteins which dissolve in contact with water to form a gel. In the body, mucous lubricates and protects mucous membrane and plays a role in immune function. Each villi, therefore, helps to keep the small intestine lubricated and actively protects from infection and contamination.
The villi also synthesize enzymes which further break down nutrients until they are small enough to be absorbed by the absorptive cells of the villi. Absorptive cells, as their name implies, absorb or take in nutrients from the nutrient-rich chyme.
Nutrients are absorbed from the fluid environment of the lumen into the internal fluid environment of the villi where they are picked up by the villi's blood or lymph capillaries and thus into their respective circulatory systems.
Absorption is not a single process but different strategies depending on what is to be absorbed. There are several possibilities or a combination of strategies.
Most essential nutrients are actively transported, with the exception of minerals which use passive diffusion and may require a carrier molecule but do not require an energy source to make the transition.
Small molecules such as electrolytes, water, small sugars and other water solubles are absorbed by passive diffusion or passive absorption.
Amino acids require both energy and a carrier to make the leap so they are actively transported.
The energy needed to power absorption, when energy is required, is provided by a chemical called adenosine triphosphate (ATP) which is produced in the body's cells by their mitochondria. Mitochondria are little power generators in every cell and there are many in each cell. The energy the mitochondria need to synthesize ATP is provided by food, sparked by the oxygen breathed in and transported around the body by the red blood cells and their iron component.
If you would like to see animations of the absorption processes, click on the graphic to the right for a good selection:
Here is a more magnified view inside the tips of those villi:
Thanks again to Henry Gray, we have a cross section of a single villus to better visualize the workings of this bustling community.
When we view this graphic, we need to imagine interstitial fluid bathing all the cells. Do you see those goblet cells emitting mucin? And do you see all those leukocytes, both in the epithelium (villi 'skin') and below the epithelium (inside the villi)? Leukocytes are white blood cells and the villi are well stocked, especially with phagocytes like neutrophils and monocytes who attack potential invaders and gobble up debris. If you have not seen this old video of a neutrophil chasing a bacterium, it is definitely worth watching and will, I think, alter your perception of white blood cells significantly. Click the cat chasing the mouse, the video link will open on a new page.
Finally we have the illustration below to bring the various concepts together. There is the mesentery, the fold of the peritoneum which supports and anchors the small intestine. We see the enteric nervous system and the various layers of the structure, we clearly see the villi carpeting the interior.
That graphic points to an especially important aspect of the enteric nervous system – communication between the ENS and the gut occurs across the lining of the gut without the ENS physically entering the lumen. This communication utilizes neurotransmitters, particularly serotonin, in addition to physical pressure registered by the ENS. A majority of the body's serotonin is produced in the gut, serotonin which along with other neurotransmitters signals not only the ENS but also keeps the brain informed of the gut's doings.
Remember that the lumen is the space in the intestines, the channel where the chyme flows.
No one needs to pass a test on all that material above. Some readers will be interested in the details, others will not. The main message is that this is a complex environment, not a simple tube, and a very important factor in the life and health of a cat.
The health of the cat depends on the health of the villi who themselves depend on good nutrition and nonexposure to toxic substances. Villi also require the physical passage of food in a use-it-or-start-to-lose-it response. In a cat (or a human or any species with small intestinal villi) who is not eating, the villi shrink. They can grow again, they can recover, but in the meantime health is at risk because both the gut barrier function and the ability to absorb nutrients are compromised.
Maintained by new cells migrating from the bottom up to their tips, in a continuous process, the old cells at the tips of the villi die and fall into the lumen where they are digested and anything reuseable is recycled, the rest passing out in the poop. Think of that cellular migration as a bucket brigade. The migration of cells depends on nutrition and the physical passage of food and will slow surprisingly quickly if meals are missed or a cat is not eating, as though not only is there no water for the bucket brigade but the buckets are missing.
The villi are also subject to damage by toxic substances, intestinal parasites, food allergies, food poisoning and anything that limits or cut off the blood flow or oxygenation to an area. Fortunately the fast cellular turnover in the villi means the gut wall has great healing capability when negatives are eliminated and a healthful diet is served and eaten regularly. Regularly for a cat means small frequent meals.
One of the important aspects of the small intestine is that it evolved to transport liquidy contents, not solid objects. The chyme delivered into the small intestine from the stomach is liquidy, not solid. Despite the marble run analogy in The First Lesson, solid objects do not flow along well. They are stiff and unyielding and the small intestine consists of many twists and turns. Trying to inch too large a solid object along that path is like trying to get a hefty piece of furniture around a tight corner or wrestled down a narrow hallway. The small intestine is flexible and expandable and it works hard at its job but still, it was designed for chyme, not hefty solid objects such as formed hairballs which might manage to escape the stomach downwards rather than get vomited up and out.
A linear foreign object, such as a string, is particularly risky because as it moves along, its length may occupy more than one turn of the small intestine and could cinch several sections together in a strangle-hold, cutting off the blood supply to the area.
Bit by bit, as chyme reaches the end of the small intestine, it is released into the large intestine through the ileocecal valve, the sphincter or gate between the small and large intestines. By then, most everything which counts as a nutrient for the cat has been absorbed upstream and what’s left, besides water, are the parts of the diet which can't be digested. These remainders are called dietary fiber and play a feature role in the large intestine.
We leave digestion and absorption at this point since our focus is in the gut, not inside the body. Readers interested in internal metabolism can find a remarkable amount of information on the web, in books stores and in the public library.
Like the small intestine, the large intestine is anchored by its own mesentery which is called the mesocolon.
The first part of the large intestine is called the cecum. In cats, this section is very small in comparison to most other species and its function in cats is presently unknown.
The large intestine or colon wraps around the coiled-up small intestine. In humans, the ascending colon runs up the right side of the lower abdomen, the transverse colon travels across the abdomen, and the descending colon travels down the left side of the body where it turns a corner into the middle of the body and descends to the anus. There are many graphics illustrating this on the web.
In the cat, the arrangement is similar except a cat is much narrower across the midriff than a human. A cat’s ascending colon is short in comparison and the transverse colon is also proportionally short. The descending colon runs horizontal because cats walk on all fours, but it is still called the descending colon as it descends to the anus. There is no last corner to turn. Positioned under the spine, the descending colon runs straight to the anus below the tail of the cat.
When the chyme reaches the large intestine or colon, electrolytes and excess water begin to be drawn off from the soupy chyme and resorbed into the body. The large bowel makes poop out of soup! If water and electrolytes weren't reclaimed, we'd all be dehydrated by constant diarrhea.
The large intestine or bowel is also where fermentable dietary fiber in the chyme is fermented by the trillions of gut bacteria who live in the bowel. These and other processes, discussed in the next chapters, result in what we call feces or stool or poop.
Formed poop is stored until time to move the bowels. Pressure from the poop, moving into the rectum and pressing from the inside out on the nervous system of the bowel, the enteric nervous system, signals the bowel when it is time to move. At this point in the process the signals from the enteric nervous system coordinate with nerve cells in the corresponding section of the spinal cord, of the central nervous system, to ensure that the spine cooperates in the best position for the poop to exit successfully. The proper squatting position facilitates pooping. Cats who have arthritis or who have suffered an injury to that area may experience constipation if the ENS and CNS are not able to coordinate at that juncture.
As stool moves from the rectum through the anal canal, the inner and outer anal sphincters relax and the canal shortens, effectively pulling this little 'storm porch' up and over the exiting stool as the rectal muscle exerts peristaltic contractions in response to the stool. The inner sphincter consists of smooth muscle without voluntary control but the outer sphincter is striated muscle and under voluntary control. And there it is, poop in the litter box!
Please do not skip over the next sections. It is important to understand the function of water and fiber in the whole process, and what poop is and is not, so that constipation can be treated and prevented as effectively as possible. It is especially important to read about the gut bacteria and their role in bowel health. The best treatment for a constipated cat is a caring human who has a general understanding of gut workings.
“Cats require purity and simplicity.” – SEM